1. Field of the Invention
The invention relates to the field of control devices for activating and deactivating space heating means and the like to maintain a desired temperature. In particular, the invention concerns a controller coupled to activate a supplemental heater in a heating system having a heat pump as the primary source of heat, and a two stage room thermostat with switched outputs representing two room temperature setpoints. The heat pump is controlled to seek the higher room temperature setpoint. The controller senses and responds to the temperature of duct air from the heater, and adjusts automatically the duct temperature setpoint at which the supplemental heater is activated.
2. Prior Art
Heat pump systems use a refrigerant to carry thermal energy between a relatively hotter side of a circulation loop, where compression of the refrigerant by a compressor raises the temperature of the refrigerant, to a relatively cooler side of the loop at which the refrigerant is allowed to expand, causing a temperature drop. Thermal energy is added to the refrigerant on one side of the loop and extracted from the refrigerant on the other side, due to the temperature differences between the refrigerant and the indoor and outdoor air, respectively, to make use of the outdoor air as a thermal energy source. Heat Dump systems are also applicable to other heat sources and heat sinks such as ground source thermal sources/sinks, waste heat from industrial processes and the like, but are discussed herein with respect to air-to-air space heating, as an example.
The heat pump can be bidirectional. Suitable valve and control arrangements selectively direct the refrigerant through indoor and outdoor heat exchangers so that the indoor heat exchanger is on the hot side of the refrigerant circulation loop for heating and on the cool side for cooling. A circulation fan passes indoor air over the indoor heat exchanger and through ducts leading to the indoor space. Return ducts extract air from the indoor space and bring the air back to the indoor heat exchanger. A fan likewise passes ambient air over the outdoor heat exchanger, and releases heat into the open air, or extracts available heat therefrom.
Heat pump systems of this type are operative so long as there is an adequate temperature difference between the refrigerant and the air at the respective heat exchanger to maintain a transfer of thermal energy. For heating, the heat pump system is efficient provided the temperature difference between the air and the refrigerant is such that the available thermal energy is greater than the electrical energy needed to operate the compressor and the respective fans. The temperature difference generally is sufficient for efficient cooling, even on hot days. However, for heating when the outdoor air temperature is below about 25.degree. F. (or -4.degree. C.), the heat pump system may be unable to extract sufficient heat from the outdoor air to offset the loss of heat from the space due to convection, conduction and radiation of heat from the structure to the outdoors. A supplemental heating means is provided to supply the additional heat required to maintain the desired indoor air temperature. This is typically an electric resistance heater having elements disposed in the distribution duct downstream of the indoor heat exchanger of the heat pump, along the forced air path. It is also possible to use other forms of supplemental heaters, such a hydronic coils that carry heated water or the like.
Activation of the supplemental heater is typically controlled by an indoor (or room) thermostat, by which the occupants set a desired temperature to be maintained in the space by operation of the heating system. Conventional heat pump control systems use a two-stage-heat/one-stage-cool room thermostat. On a call for heat from the thermostat, the heat pump compressor and fans are activated to extract heat outdoors and to release the heat indoors. The heat pump operates until the indoor temperature reaches the thermostat setpoint and then is deactivated. If the heat loss of the structure is greater than the capacity of the heat pump, which occurs when outdoor temperatures drop, the indoor air temperature cannot be raised by the heat pump to the desired temperature. The indoor temperature continues to drop.
The room thermostat has a second switching means that is operated at a temperature slightly lower than the desired temperature at which the first switching means is operated. Conventionally, when the room temperature falls to the second setpoint defined by the thermostat, power is supplied to the supplemental heater. The supplemental heater supplies the additional heat needed to bring the indoor temperature up to the second setpoint temperature. This meets the objective of maintaining the room temperature, but requires power for the supplemental heater to replace the needed heat energy that could not be obtained from the outdoor air.
The conventional two stage heat control as described, causes wide swings in the temperature of the air emitted into the structure by the heat pump system. The duct air temperature may be as low as 80.degree. F. (27.degree. C.) when the heat pump is operating alone and the outdoor temperature is relatively low. Assuming the supplemental heater is arranged to heat the duct air, when the supplemental heater is operating together with the heat pump the temperature may rise to as high as about 125.degree. F. (52.degree. C.). This temperature swing occurs suddenly, upon activation of the supplemental heater. Other forms of supplemental heater also cause temperature swings associated with their output means.
A duct temperature of 80.degree. F. is uncomfortably cool for the occupants, particularly when they are directly in the path of air emitted from a register. A duct temperature of 125.degree. F. is inherently inefficient, because the duct air is so much warmer than the indoor air (typically controlled to a target temperature of 68.degree. to 72.degree. F., or 20.degree.-22.degree. C.) that temperature stratification occurs. The warm supplementally heated air rises to the ceiling, where the temperature can be as much as 8.degree. to 10.degree. F. warmer than near the floor. Not only is the warm air near the ceiling out of the area where it is needed for the comfort of the occupants, but in addition, stratification increases the temperature differential across the insulation in the ceiling and along the tops of the walls of the structure, leading to increased heat loss from the structure.
It would be desirable for improving the comfort of occupants and tier conserving energy, to provide a control system that provides a closer control on the operation of the supplemental heater, for example an electrical resistance heater. Advantageously, the control should maintain a comfortably warm duct temperature even as heat pump capacity and room temperature are sinking toward the level at which the supplemental heater is needed, but without wasting energy by unnecessary operation of the supplemental heater, and without causing bursts of hot air leading to stratification.
It would be possible to provide supplemental heating means with a proportional control, operable to activate the supplemental heater at a variable power level as a function of loading requirements. However, such a system is relatively more complex than an on/off controller, and makes more frequent use of the supplemental heater than would an on/off control. This is undesirable, because heat from the supplemental heater is more expensive than heat from the heat pump, and the full capacity of the heat pump should be utilized before the supplemental heat is activated at all.
When heat loading requirements change (for example if the sun begins to shine through the windows or use of a cooking stove adds indoor heat), a proportional control can react by reducing the power applied to the supplemental heater. However, the control is not adaptive in that it responds only to the room temperature. Moreover, a proportional control inherently applies the highest power level to the supplemental heater when the room temperature is at the lowest. In that case the proportionally controlled system has the same disadvantage as an on/off control with respect to heat burst operation. i.e., temperature stratification and increased heat loss when the heat pump is operating constantly but is failing to meet the load.